the WTA Finals.
The three-time defending champion made it 16-straight wins at the season-ending championships by beating Ana Ivanovic 6-4, 6-4 in Monday's tournament opener, shaking off the injuries that forced her out of her previous two tournaments and the off-court distraction of disparaging comments about her from Russian tennis official Shamil Tarpischev.
Williams is now five wins behind Martina Navratilova's all-time record of 21 in a row at the tournament, although she won't have enough matches to catch up this year.
"I just found out last time that I was 15 matches and I was like, 'Oh boy, not another streak,'" Williams said. "So now I'm 16 and, gosh, here we go again."
Williams looked right at home in Singapore's brand new indoor stadium, with the tournament having moved from Istanbul this year, despite having pulled out of her previous two tournaments in China through illness and a knee injury. She also showed no signs of having been preoccupied with an off-court issue involving comments by top Russian sports official Shamil Tarpischev.
He has been heavily fined and suspended by the WTA for referring to Serena and elder sister Venus as "the Williams brothers." He claimed the line was out of context and meant as a joke, but Williams said it was sexist and racist.
In the other Red Group match, Simona Halep beat Eugenie Bouchard 6-2, 6-3 in a meeting between the only two newcomers in the eight-player field.
On Tuesday in the White Group, Maria Sharapova will take on Caroline Wozniacki and Petra Kvitova will face Agnieszka Radwanska.
The top-ranked Williams was shaky in her opening service game, having to save two break points, but soon settled and took a 4-1 lead. Ivanovic got back on serve at 4-4 and had a crucial break opportunity to go up 5-4 but missed a volley.
"That was a huge point but it was actually quite a hard volley, the ball was dropping fast and I didn't come in fast enough," Ivanovic said. "I kind of got down a little bit on myself, then I rushed a little bit that (next) service game, so that was a little bit disappointing."
In the final game of the opening set, Ivanovic fell behind 0-40 and eventually dropped serve on a double-fault — one of seven she made during the match.
Ivanovic came from a set down to beat Williams at the Australian Open, and looked capable of a similar comeback when she broke in the opening game of the second set, but the American immediately broke back.
Williams served 12 aces and improved her record against Ivanovic to 8-1 despite her injury-interrupted preparation.
"I felt the knee a little bit, but compared to what it was in Beijing, it just feels so much better," Williams said. "I was up 4-1 in the first set and cruising and she does what she does best. I felt my energy level got a little low and then I just tried to do everything I can to bring my energy level up to just to fight."
Williams will next face Halep on Wednesday.
In the second match, Bouchard managed to win only four points against Halep's first serve and committed 30 unforced errors against only 11 by her opponent.
"Going for it is always my game style but today there was definitely a lack of consistency," Bouchard said. "I didn't feel match sharp, but I feel like I can build on this and I still have another match, which is a bit of a positive."
Not a lot was expected of Halep at this tournament, but she already has one win ahead of the match against Williams.
"Maybe I can believe," Halep said. "I have the chance to win this tournament, but I don't want to think about this."